RE: How many decoys are really needed for goose on farmland?
The number you need has a lot to do with how much competition you have from fellow hunters. If you're in an area that doesn't get much pressure, 2 dozen full bodies will do fine.
If, however, there are other hunters around, you will need to increase that number substantially. It's not uncommon to see 100-150 fullbodies.
Good luck,